1. Field of the Invention
The present invention relates to an ultrasonic motor or a vibration driven motor which bends and vibrates a pencil-type vibrator which holds both sides of an electro-mechanical energy conversion element such as a piezo-electric element thicknesswise by supplying an energy to the electro-mechanical energy conversion element to cause a rotary motion such as a circular or elliptic motion of a mass point so that a movable body pressed to the vibrator is frictionally driven.
2. Related Background Art
In a prior art ultrasonic motor, a travelling bending vibration is caused in a ring-shaped metallic vibrating elastic member to drive a movable body by a frictional force. This type of ultrasonic motor has been used in an auto-focusing mechanism of a camera.
However, in this type of ultrasonic motor, since the vibrating elastic member is of ring shape, a cost of a unit including a pressing mechanism to create a frictional force is high and it is disadvantageous in terms of cost in an application which does not require hollowness (ring shape).
As an ultrasonic motor of a bar type such as pencil type which has a simple pressing mechanism, a motor shown in FIGS. 9 and 10 has been proposed.
In FIGS. 9 and 10, a symbol A denotes a vibrator of a pencil-type ultrasonic motor or a vibration wave-motor. It comprises a pencil type front vibrating elastic member 1, a cylindrical rear vibrating elastic member 2, doughnut-shaped piezo-electric plates 3 and 4 as electro-mechanical energy conversion elements provided between the front vibrating elastic member 1 and the rear vibrating elastic member 2, and electrode plates (not shown) for applying an AC voltage to the piezo-electric plates 3 and 4, provided between the piezo-electric plates 3 and 4. The piezo-electric plates 3 and 4 and the electrode plates are held and secured by bolts 6 between the front vibrating elastic member 1 and the rear vibrating elastic member 2.
The piezo-electric plates 3 and 4 are polarized with different polarities symmetrically about a cross-section which passes through an axis, and the plates 3 and 4 are shifted by 90 degrees along a direction .theta..
When AC voltages V.sub.1 and V.sub.2 having frequencies close to specific bending vibration frequency of the vibrator are applied to the piezo-electric plates 3 and 4, the piezo-electric plates expand or shrink thickness-wise to cause the bending vibration in the vibrator. If the AC voltages V.sub.1 and V.sub.2 have the same amplitude and frequency and a 90-degree phase shift therebetween, the vibrator A makes a circular motion like a rope in a ropeskipping herein (after called a ropeskipping vibration) around an axial center of the vibrator. In other words, when a plurality of bending mode vibrations having a predetermined phase difference therebetween are caused by the piezo-electric elements 3 and 4 in a plurality of planes of the bar-shaped elastic member, the rotary motion is caused in the particles of the elastic member. By inverting the phases of the AC voltages V.sub.1 and V.sub.2 , the forward and backward rotations of the circular motion are attained.
A symbol R denotes a rotor coaxially fitted to the axial center l of the vibrator A. One fitting end thereof is pressed to a sliding area B of the vibrator by a spring force of a spring 5 and it is rotated by frictional drive by the vibration caused by the vibrator A. The spring 5 is resiliently loaded between a tip end of the bolt 6 and a spring post 8 which fits to a thrust bearing 7 having a flange.
As a method for supporting the vibrator A, it has been proposed to provide a flange on a side wall of the vibrator and support the flange by a low friction material. In this method, in order to support the vibrator without restricting the vibration of the vibrator, a fixed area must slip. As a result, an energy loss is created by the friction.